Microwave circuits are commonly used in the telecommunications field, where they are a constituent of transmission/reception chains, which include in particular antennae and radio-frequency analog circuits for signal processing (filters, impedance matching, amplification).
High levels of compactness, efficiency and integration are therefore looked for in respect of these circuits, and particularly in respect of the radiating elements built into the design of the antennae and of the resonating elements useful for the filters and the impedance matching. To satisfy such requirements, substrates need to be designed that have the special feature of possessing high permittivity and permeability values in the micro-wave frequencies, in other words frequencies of between 1 and 20 gigahertz. Indeed, said materials can be used to meet the needs for circuits with high levels of compactness and therefore of integration, and for substrates that are functionalized, in terms for example of band gap, “left-hand” properties and frequency agility.
A plurality of paths have thus been explored in relation to the constituent materials of microwave circuit substrates, such as for example using so-called “high K” dielectric materials or adapting low frequency piezoelectricity techniques to the microwave field.
However, there is no material known today that possesses both high permittivity and high permeability in the microwave frequency field.
Ferrites have certainly been the subject of intensive research to this end, but their permeability by permittivity product is difficulty reaching a value of a hundred in the micro-wave frequencies, which proves unsatisfactory.
One solution to obtain the looked-for properties therefore comprises combining high permittivity dielectric materials with high-permeability ferromagnetic materials in the frequency band of interest. This combination has not hitherto been possible since the development of a high-permittivity dielectric material, such as a stoichiometric oxide, like HfO2, Ta2O5, BaTiO3 or SrTiO3 for example, requires the use of a method involving significant (deposition or anneal) temperatures, and typically above 500° C., while ferromagnetic materials (such as NiFe, CoZrNb, FeHfN, FeCoB etc.,) cannot withstand said temperatures, without seeing their magnetic properties drastically reduced. There is therefore currently a technical incompatibility preventing the manufacture of a composite material of high permittivity and high permeability.